In electronic prepress systems, images to be printed by offset printing are scanned from photographic sources, digitized, assembled, and edited electronically at a workstation. The images are then transmitted to a raster image processor (RIP) for half-tone screening and image rasterization. The RIP image, or rasterized image, to be printed is then transmitted from the RIP to an imagesetter for photographic or film recording onto a medium such as paper, film, or a printing plate.
An imagesetter includes a supply of unexposed photosensitive media, a recording support surface, and an image exposing system for forming the image to be recorded according to the RIP image data. The image exposing system may employ a laser beam, a cathode ray tube (CRT), an LED emitter, or the like as a radiation source. The media passes either as single sheets or from a supply roll as a web to the recording support surface at which point the photosensitive media is exposed by the radiation source, forming a latent image on the media. Numerous images may be recorded on the web consecutively. The exposed web is then advanced for transfer to a media processor where chemical processing occurs.
Three inks, yellow, magenta, and cyan, are used to print color images. Often black ink is also used. The links are printed in small dots, sometimes overlaid, in varying amounts to create the desired colors when viewed. Thus, three or four black and white separation films must be imaged, one for each color.
In the printing process, the films are overlaid and must be aligned accurately to ensure a good quality image. Toward this end, registration openings or holes are punched in each film to serve as an alignment guide. The location of each pixel on each film is determined with respect to the registration openings.
In prior art punch assemblies, the leading edge of the media is fed into a punch assembly, also referred to as a head punch assembly, in the imagesetter. The punches are forced through the media and held while the media is imaged. After imaging, the punches are retracted from the media. In prior art punch assemblies, the punches arc biassed toward the retracted position by a spring mechanism. Upon release of the punching force, which must be sufficient to overcome the spring bias force, the spring mechanism causes the punch to retract. The spring mechanism, however, has been found to permit binding of the punches in the media.